The present invention relates to surface relief-volume reflective diffractive structures, and methods and systems for producing such structures, and more particularly to surface relief-volume reflective diffractive structures for easily recording replicable, single and multicolor diffractive images, and methods and systems for producing such structures.
Conventional surface relief diffractive structures generally include a shallow edge profile, e.g., 100-300 nanometers, that is sinusoidal in shape and has a pitch, or spacing between maxima of the sine wave profile, of around one micrometer. Such a shallow profile generally enables relatively easy mechanical replication of the structure. The shallow profile of such surface relief structures resulting from an initial recording of an object's image, using continuous wave laser light, in the photosensitive material diazonaphthoquinone, commonly known as AZ photoresist, or simply, photoresist. The recording geometry generally included two beams of light, in which one beam, corresponding to light from an object (i.e., “object” beam) interferes with a second beam (the “reference” beam). Both beams are initially incident on the same side of the recording medium such that the resultant interference fringes are mainly perpendicular to the surface. This exposure technique is referred to as an “off-axis” exposure, since both beams are at an angle with respect to the normal to the surface. Photoresist material has the property that when exposed and developed, the material is removed (etched) from the top surface downwards in proportion to the exposure intensity. For a typical hologram, such exposure intensity is sinusoidal in nature such that the resulting surface of photoresist has a sine wave variation in depth. One technique to mechanically replicate the developed photoresist, the photoresist is coated with a layer of conductive metal and converted into a hard nickel replica through electroplating techniques. The hard nickel replica can then be used to impress the surface pattern into plastic using, for example, heat and pressure, or through ultraviolet casting. The typical diffraction spectrum from a hologram formed in this way covers the realm of the entire visual range from blue to red, or about 400 to 700 nanometers (thus, the resultant holograms are referred to as “rainbow holograms”). It is not feasible to produce with shallow surface relief structures single, individual, color holograms.
Individual colors can be made holographically with a different type of diffractive device called a volume reflection hologram (or sometimes volume phase reflection hologram). In this case, the object and reference light beams are brought into the recording medium from opposite sides, and the interference structure that forms upon development is a set of planes separated by a distance half the wavelength of the incident light divided by the index of refraction of the medium. The spacing in this case is very fine. For example, with green light having a wavelength of 500 nanometers and a recording medium having an index of refraction of n=1.5, the fringe spacing is d=500/(2×1.5)=167 nanometers. For this structure a different medium than photoresist is generally used. Typical recording materials include dichromated gelatin (DCG), photopolymer, and silver halide. These diffractive planes are not surface relief structures, but rather include regions of different indices of refraction induced by the exposure. In reconstruction of the recorded image with incident white light, a small portion of light is reflected from each of the interference planes, and because of the half-wavelength spacing, the reflected light is coherent with reflection from all the other planes. The coherently reflected light is viewed as a single color that is the same as the color of the original recording light. The remaining colors are incoherently scattered out of the field of view. Because the fringe structure includes of a set of parallel planes distributed throughout the medium, there is no surface relief structure that can be mechanically replicated. Thus, replication of these volume holograms is performed optically, using, for example, a laser exposure on a production line.
Another technique for recording holographic images is one predicated on using a recording geometry similar to that used for recording volume holograms (i.e., directing the object beam and reference beams from opposite sides of the recording medium) to record the interfering and non-interfering patterns in a thick layer of photoresist, so that two sets of interference fringes are formed that are perpendicular to each other. When this recording is developed, etching of the photoresist proceeds in a manner similar to the recording of thin-layer photoresist media due to the off-axis exposure. However, because of the additional planes produced from the counter propagating beams, the resultant sine wave profile becomes modified into a stepped profile, with the steps separated by a half-wavelength, as with the volume case. In this case the profile has the appearance, from the edge, of a stepped pyramid (thus inspiring the name given to the structure of an AZTEC structure which due to the resemblance of the resultant structure to Aztec temple pyramid, and also because Aztec is a useful mnemonic of the recording technique diazo photoresist technology). Resultant stepped-pyramid structures can be mechanically replicated in a manner similar to the mechanical replication of shallow surface relief structures. Such stepped-pyramid structures also produce, upon illumination by a light sources, a single-colors in reflection (i.e., each point of the reflected image includes a single color, related to the original recording color, rather than a rainbow of colors produced when a shallow surface relief structure is illuminated.
While Aztec structures formed in this way do demonstrate properties of volume holograms, there are some significant differences in the diffraction characteristics. Because the recording is done by two separate exposures, one surface and one volume, the diffracted light has properties of both. The stepped structure does indeed produce single color reflected light. However, because it is a surface relief structure, it also produces rainbow reflected light as well. The single color light appears primarily on-axis, while the rainbow light appears predominantly off-axis.
Another feature of conventional holographic recording techniques and systems is that the recording of holographic images into photoresist layers requires use of blue or shorter wavelength light, and thus many of the recording geometries of conventional system produced images that are restricted to the blue end of the visible spectrum.